1. Field Of The Invention
The present invention relates to a novel random copolymer containing hexagonal imide units, a process for the preparation thereof. This copolymer and is useful as a substrate for an optical disc. More particularly, the present invention is concerned with a thermoplastic random copolymer which is colorless and transparent as well as having excellent heat distortion resistance, heat decomposition resistance, mechanical strength and oil resistance, and hence can be suitably employed as parts for light electric apparatus, engineering plastics, lenses, optical fiber cables, etc. Further, the present invention is concerned with a process for the preparation of a thermoplastic random copolymer. Yet further the present invention is concerned with a substrate for an optical disc, such as a digital audio disc, a video disc, a disc which is capable of being directly read after recording and the like. The optical disc substrate is especially suited for a high-density information recording medium for use in a recording and playback apparatus. This apparatus converts analogue information into digital information and records the digital information in a recording medium at a high packing density by means of a laser beam, and which consequently provides substantial convenience and advantage. Hence this apparatus has become the object of public attention.
2. Discussion Of Related Art
In the field of parts for light electric apparatus, engineering plastics, optical fiber cables, optical disc, etc., there is a strong and increasing demand for resin materials which are colorless and transparent as well as having excellent heat distortion resistance and other desirable physical properties.
A polycarbonate resin is colorless, its transparency is good and it has excellent heat distortion resistance. However, even though the transparency of this resin is good, its transparency is still insufficient. Thus, it is not employable as a material in many fields, especially those in which high transparency is required.
Polymethyl methacrylate (hereinafter, often abbreviated as PMMA) is pre-eminent in colorlessness and transparency as compared to other thermoplastic polymer resins. However, this resin is deficient in heat distortion resistance and in heat decomposition resistance.
Heretofore, there have been various proposals for improving the heat stability of PMMA while maintaining the other excellent properties thereof such as optical characteristics.
In one proposal, methyl methacrylate (hereinafter, often abbreviated as MMA) monomer is copolymerized with a comonomer. Examples of such comonomers include unsaturated dicarboxylic anhydrides, e.g. maleic anhydride; aromatic vinyl compounds, e.g. .alpha.-methylstyrene; maleimide derivatives, e.g. N-phenylmaleimide and N-o-chlorophenylmaleimide. However, this proposal has a serious drawback in that when an unsaturated dicarboxylic anhydride or an aromatic vinyl compound is used as a comonomer, the copolymer obtained is still characterized with an insufficient degree of heat stability and thus it would readily decompose in a molding step at 250.degree. to 280.degree. C. When N-phenylmaleimide or N-o-chlorophenylmaleimide is used as a comonomer, the copolymer obtained is undesirable in that it is likely to be discolored.
In a proposal previously made by the present inventors, hexagonal anhydride units of the formula ##STR2## are formed in the molecules of a copolymer obtained from MMA, styrene and at least one member selected from methacrylic acid and t-butyl methacrylate. The copolymer thus obtained has excellent heat distortion resistance and heat decomposition resistance. The above-mentioned hexagonal anhydride units are formed by heat-treating the copolymer under reduced pressure. In the heat treatment of the copolymer, in the case in which the copolymer is obtained from MMA, styrene and methacrylic acid, the following two kinds of hexagonal anhydride unit-forming reactions occur: (1) a dehydration reaction between two adjacent methacrylic acid units, and (2) a reaction between a methacrylic acid unit and the adjacent MMA unit being accompanied by the elimination of methanol. At the heat treatment of the copolymer, when the copolymer is obtained from MMA, styrene and t-butyl methacrylate (hereinafter, often abbreviated as t-BMA), the following two-stage hexagonal anhydride unit-forming reaction occurs: (3) a reaction in a t-BMA unit being accompanied by the elimination of isobutene and the formation of a methacrylic acid unit and the subsequent reaction of the thus formed methacrylic acid units would occur in the same manner as described with respect to reactions (1) and (2) mentioned above.
In regard to above reactions (1) to (3), reactions (1) and (3) proceeded smoothly (D.H. Grant and N. Grassie, Polymer 1960, 1(2), 125). However, reaction (2) is disadvantageously time-consuming as compared with reactions (1) and (3). Therefore, it is difficult to practice the method by the use of conventional vent extruders such as those disclosed in European Patent Application Publication No. 0 076 691.
In order to eliminate the above problem in production, the present inventors further proposed a commercially advantageous method for producing a polymer containing MMA units and hexagonal anhydride units. In this method, methacrylic acid and/or t-BMA is copolymerized with MMA by continuous bulk polymerization or continuous solution polymerization, and, subsequently, the copolymer produced is continuously heat-treated at 200.degree. to 300.degree. C. under reduced pressure for 10 minutes or more. This method is advantageous from the viewpoint of commercial production. However, the copolymer obtained by this method is deficient in heat distortion resistance and heat decomposition resistance.
Therefore, an attempt has been made to increase the glass transition point of the polymer containing MMA units so as to provide a polymer having excellent heat distortion resistance and would be usable under high temperature conditions.
As to polymers with improved heat distortion resistance and heat decomposition resistance, there have been proposed various polymers having hexagonal imide units of the formula ##STR3## wherein R' is a hydrogen atom, an alkyl group, etc.
U.S. Pat. No. 4,246,374 discloses a method in which a polymer containing at least 80 wt % MMA units is reacted with ammonia or a primary amine in an extruder to convert the polymer to one which contains hexagonal imide units. However, this method requires a high temperature in an extruder, the temperature being in the range of from 300.degree. to 375.degree. C. Due to the high temperature, the extruded polymer resin is discolored to a yellow color. Although it remains transparent, it is still not usable as a material which is required to be not only colorless but also transparent. Further, the water absorption of the polymer obtained by this method is undesirably high. Moreover, the molecular weight of the polymer is undesirable in that it is likely to be lowered during the reaction with ammonia or a primary amine, leading to a lowering of the mechanical strength of the polymer.
European Patent Application Publication No. 0 076 691 discloses a method in which a copolymer containing at least 50% by weight, preferably 80 to 90% by weight, of units derived from acrylic acid and/or methacrylic acid, 5 to 20% by weight of styrene units and 1 to 10% by weight of MMA units is introduced into an extruder and heat-treated to form hexagonal anhydride units in the molecules of the copolymer, and the thus modified copolymer is then introduced into another extruder and reacted with ammonia or a primary amine to convert part or all of the hexagonal anhydride units in the molecules of the copolymer to hexagonal imide units. As is apparent from the above, the copolymer to be modified by the method contains acrylic acid units or methacrylic acid units in an amount as large as 50% or more and MMA units in an amount as small as 1 to 10%. Therefore, the modified copolymer produced by the method contains more than 50% by weight of hexagonal anhydride and/or hexagonal imide units. Due to such a high proportion of hexagonal units, the copolymer has excellent heat distortion resistance but its water absorption is undesirably high. Further, the melt-flow properties of the copolymer are poor, which leads to poor molding characteristics. The poor melt-flow properties can be improved by lowering the molecular weight of the copolymer. However, the lowered moleculer weight leads to a low mechanical strength of the copolymer. Therefore, the use of this copolymer as a molding material is undesirably limited.
UK Patent Application Publication No. GB 2 101 139 A discloses a method in which a molten methacrylate or acrylate polymer is continuously imidized in a tubular reactor. This method is only a modification of the above-mentioned method disclosed in U.S. Pat. No. 4,246,374, and the polymer obtained by this method is defective in that the water absorption of the polymer is high.
On the other hand, in the field of optical discs, there have been proposed various polymers for use as materials for substrates for optical discs.
An "optical disc" used herein relates to an information disc which is read optically in reflection, and includes for example, a digital audio disc (DAD), a video disc (VD), a so-called direct read after write (DRAW) disc and a so-called erasable direct read after write (E-DRAW) disc.
In a DAD, audio information which has been finely divided into signals and converted into a binary number system of "0" and "1" is recorded, on a metal layer capable of reflecting a laser beam, as a relief structure having a crenellated profile of areas situated alternately at a higher and a lower level (information bits) sometimes termed blocks and pits. The relief structure is read by means of a laser beam, and the signals of the laser beam, according to the binary number system, are converted into electrical signals and then played back as sound. In general, such a disc having a relief structure of a metal layer capable of reflecting a laser beam is obtained by molding a transparent resin into a disc having a relief structure and forming on the relief structure surface a metal layer by vacuum evaporation or the like.
Image information can likewise be recorded as a relief structure on a metal layer capable of reflecting a laser beam to give a VD.
Computer programs and data can also be recorded on an optical disc as relief structures to give an optical disc which is usable as an information disc of computer programs and data. Discs such as information discs for computers or a disc which is capable of being directly read after writing, or recording, (DRAW disc), are especially attracting attention. Further, an optical disc which is capable of being written on and erased [erasable direct read after write disc (E-DRAW disc)]is now being developed. In the field of DRAW discs, there are employed various types of recording layers. Examples of such recording layers include (1) a layer in which holes are formed by irradiation of a laser beam, (2) a layer in which formation of bubbles by irradiation of a laser beam is utilized, and (3) a layer in which the magnetic direction is changed by irradiation of a laser beam. The recording layer of the above type (3) is usable for E-DRAW discs.
For reading the relief structure in the above mentioned various optical discs by means of a laser beam, the laser beam interference, which is caused by the phase difference between laser beam directly incident on a detector and a laser beam traversing the substrate and reaching the detector, must be detected. Therefore, the resin to be used as the material for a substrate for optical discs must satisfy the following requirements:
(1) the resin must have a high laser beam transmission;
(2) after the resin has been molded into a disc, the orientation of the molecules in the resin must be small so that the double refraction of the laser beam is small;
(3) after the resin has been molded into a disc, the refraction index of the resin must not vary over the disc;
(4) the resin must not contain contaminants;
(5) the resin must have good heat distortion resistance;
(6) the resin must be susceptible to good vacuum evaporation of a metal;
(7) the resin must have good molding properties and can be molded into a disc with sharp pit pattern;
(8) the resin must have good adhesion to a recording layer,
(9) after the resin has been molded into a disc, the thickness must be uniform over the disc; and
(10) after the resin has been molded into a disc, the disc must be stable in respect to freedom from warp with the passing of time.
In addition to the above requirements, the resin should not contain impurities such as the polymerization solvent, and should not hydrolyze.
Heretofore, polymethyl methacrylate (PMMA) has been used as a material for substrates for optical discs. However, the water absorption of PMMA is high and, hence, an optical disc utilizing PMMA as the material for its substrate would in time become warped, due to the absorption of water by the substrate. In order to eliminate this drawback, there has been proposed an optical disc in which a substrate made from a PMMA resin is covered with a resin having good barrier properties to water, such as polyvinylidene chloride resin. However, for preparing such a disc, a troublesome step is required.
A polycarbonate resin has also been used as a material for a substrate for optical discs. However, the substrate made of this resin is defective in that it inevitably contains impurities such as chromium and dichloromethane which deteriorate the recording layer. The contamination of the substrate with chromium is caused during the molding of the resin which is usually conducted at high temperatures. The contamination of the substrate with dichloromethane is caused by the dichloromethane used as the polymerization solvent which has not been removed. Further, the substrate made of the resin would in time become hydrolyzed, which would lead to the deteriation of the adhesion of the substrate of the recording layer.
Further, there have been proposed optical discs in which a copolymer of methyl methacrylate and styrene (methyl methacrylate/styrene =60/40 or 30/70 by weight) is used as the material for substrates (see Japanese Patent Application Laid-Open Specifications Nos. 57-33446/1982 and 57-162135/1982). However, the substrates are deficient in heat distortion resistance and the double refraction thereof is disadvantageously high.
The present inventors previously proposed an optical disc substrate made of a resin comprising a copolymer of a methacrylic acid ester and a monoalkenyl aromatic compound (see Japanese Patent Application Laid-Open Specification No. 58-88843/1983). This substrate substantially satisfies standard requirements for DADs having a diameter of 120 mm and a thickness of 1.2 mm. However, it is difficult for this substrate to satisfy standard requirements for DRAW discs having a diameter of 300 mm and a thickness of 1.2 mm. Illustratively stated, due to the increased diameter of DRAW discs, the length in which a molten resin, when subjected to injection molding, flows from the center portion to the edge portion of the mold becomes large as compared with the case of the molding of DADs, so that the orientation of the molecules is likely to occur, leading to an increase in double refraction. Therefore, it is difficult for the substrate to have a double refraction as small as 40 nm or less, which is required as the standard value for DRAW discs, and further as small as 20 nm or less, which is required as the standard value for E-DRAW discs.
The present inventors further proposed an optical disc substrate made of a resin comprising a copolymer which is obtained by copolymerizing 40 to 70 parts by weight of methyl methacrylate, 5 to 20 parts by weight of a methacrylic acid ester with a saturated aliphatic monohydric alcohol having 3 to 6 carbon atoms, and 25 to 40 parts by weight of monoalkenyl aromatic compound, and which has a solution viscosity of 3 to 10 cps as measured at 25.degree. C. with respect to a 10% solution of the copolymer in methyl ethyl ketone and a double refraction of 100 nm or less (see Japanese Patent Application Laid-Open Specification No. 59-108012/1984). This substrate is satisfactory in double refraction and water absorption. However, this substrate is deficient in the heat resistance required when forming a recording or reflection layer on the substrate by sputtering or the like, and is defective in that creep is likely to occur when a disc using the substrate is leaned and stored for a long period of time under warm conditions.
Therefore, there has been a strong demand in the field of an optical disc for resin materials which satisfy the above-mentioned requirements (1) to (10).